A flyback converter is a buck-boost converter having a two-winding inductor for isolation and non-inverted output, in which the storage and conversion of energy are realized by charging and discharging the magnetizing inductor. Generally, for achieving optimal efficiency, a universal-input flyback converter operates in a continuous conduction mode (CCM) during low line voltage and operates in a discontinuous conduction mode (DCM) during high line voltage. However, propagation delay in the flyback converter causes the maximum output power of the flyback converter to vary with the line voltage. The propagation delay is an accumulative result produced along the path through a current limit comparator, a pulse width modulation (PWM) logic circuit, a gate driver and a power MOSFET. FIG. 1 is a waveform diagram of the inductor current in a flyback converter within a switching cycle T using a conventional current limit, in which waveforms 10 and 12 represent the inductor currents at a high line voltage and a low line voltage, respectively. Due to the propagation delay Tp, both of the inductor currents still keep rising after they reach the current limit value VCL. As the rising slopes of the inductor current waveforms 10 and 12 vary with the line voltage, the differences between the peak currents Ipk thereof and the current limit value VCL caused by the propagation delay Tp in the two cases may be significantly different from each other. Assuming that the switching frequency fs is constant, and the efficiency η is constant, the output power will be determined by the energy stored in each cycle asPomax=0.5·fs·L·(Ipk2−Ivalley2)·η,  [Eq-1]where L is the magnetizing inductance of the primary coil of the transformer, and Ipk and Ivalley are the peak value and the valley value of the inductor current, respectively. As shown in FIG. 1, for a flyback converter controlled under a constant current limit, the output power at a high line voltage is far greater than that at a low line voltage, and in consequence the flyback converter requires a wide-ranged output power tolerance. It is possible that such a flyback converter will require a tolerance range up to 100%, which nevertheless tends to cause problems such as component stress or system error under over-power situation. Since over-power tests are routinely performed for flyback converter products, system designers often have to make great efforts for tradeoff between the aforesaid factors and market demands.
Due to the use of primary-side control, most of PWM controllers have trouble with output over-power control over a wide range of line voltages. Thus, it has been a common goal for PWM controllers to narrow the tolerance of output over power.
U.S. Pat. No. 6,674,656 disclosed a PWM controller which provides a time-dependent current limit VCL(t) that varies along a built-in sawtooth waveform, as shown in FIG. 2, to tighten the output power tolerance, as shown in FIG. 3. Although simple and direct, this control approach has the following defects.
First, while being one-size-fits-all, this control approach has its built-in sawtooth waveform determined according to the system parameters of the most common flyback converter products. In order to obtain better output power convergence, the parameters may be finely tuned to match the built-in waveform. However, some of the parameters, such as efficiency, EMI and thermal factors, must be compromised and thus affect primary inductance, current sense resistance, and gate driver resistance. Consequently, the output power often deviates from the original design so that system designers will have to spend more time, or additional elements be required, to deal with the deviation. Besides, it is frequently found in practice that some controllers lead to better system output convergence than others.
An even more serious problem related to the conventional control approach is that, for simplifying the circuit configuration, the current limit VCL(t) has a sawtooth waveform, i.e., a waveform having a single slope. When a system requires a relatively large primary inductance whose current slope is smaller or slightly greater than the slope of the current limit at low line voltage, the waveform of the current sense signal may never reach the waveform of the current limit VCL(t) until the maximum cycle is reached. In this case, the output voltage is out of regulation, and the function of output power limit is lost.
At last, owing to the time-dependent waveform in each cycle, it is difficult to establish a fast test for mass production, and in consequence the waveform tolerance in the datasheet is hard to define. Nevertheless, the waveform tolerance is a key factor in forecasting the tolerance of output power.